Currently marketed passenger vehicles are often equipped with a steer by wire steering device in which front wheels are mechanically decoupled from a steering wheel, and steered by a steering actuator in response to operation of the steering wheel. In a steer by wire steering device, a steering unit having a steering actuator for turning the front wheels and an operating unit including a reaction force actuator for applying a steering reaction force to a steering wheel are provided. In order to allow the steering operation to be performed at the time of a failure, the steering unit and the operating unit can be connected to each other via a clutch which is normally disengaged but can be engaged when required.
In a vehicle equipped with a steer by wire steering device, if the steering wheel is turned while the vehicle is stationary, a steering unit produces a relatively large steering torque, and the operating unit also produces a correspondingly large reaction force so that the steering device consumes a large amount of electric power. If the vehicle is an idle stop vehicle, the alternator of the vehicle produces no electric power during an idle stop operation. Therefore, it is desirable in an idle stop vehicle to prevent excessive power consumption by the steering device when the vehicle is stationary. It is therefore known to drive the steering actuator with a smaller power or stop the operation of the steering actuator while the clutch is kept engaged and the reaction force actuator is allowed to operate normally during an idle stop operation. See [0282] to and FIG. 17 of JP2014-205474A.
According to another conventional solution to this problem in an idle stop vehicle provided with a steer by wire steering device, the idle stop operation is terminated (or the engine is restarted) when the steering angle is increased beyond a prescribed threshold value. In order that the restarting of the engine may be completed before the steering wheel is turned to the maximum steering angle, the prescribed threshold value of the steering angle is determined as a value equal to the maximum steering angle minus the product of the steering angular speed and the time required for the restarting of the engine. See JP2005-351202A.